Added density fade and field is now movable.

 - Fluid now fades so that there isn't a 'white out': rate of fade
adjusts dynamically depending on how much the maximum density in the
grid exceeds a threshold.  Could be improved by using the average
density in the grid instead of d_max. Also the variance could be
calculated then the speed could be adjusted according to the uniformity
of the data...
- Fluid field position can now moved